Seismic wave sources capable of generating controlled-waveform vibration signals in natural earth media and in engineered materials and geotechnical structures offer potential advantages over conventional sources such as explosive or weight-drop impulse sources. For example, when the radiated seismic waveform is accurately known as a result of controlled-waveform source excitation at one location on the ground, seismic waves detected at other locations can be processed to take advantage of their known coherence characteristics relative to the originating source signal. The desired signals can thereby be accurately and uniquely discriminated from non-coherent interfering noise to gain a useful enhancement in detected signal-to-noise ratio. Controlled-waveform operation of the source also allows the amplitude, time duration, and frequency spectrum to be adjusted to offset certain difficult or detrimental seismic wave propagation or reflection conditions that may occur in some field applications. As a first example, improved seismic resolution may be obtained in many applications by intentionally extending the frequency range and/or boosting the amplitude of the high-frequency spectral content of the source signal. In another example, repetitive source signal operation and detected signal waveform averaging can be employed with maximum success when the seismic source amplitude is adjusted to impart forces that are below the plastic deformation limit of the ground medium. Thus, through repetitive low-force operation, the effective controlled-source energy may be increased cumulatively in relation to the number of repeated source signal sequences which, when additively combined, will result in improved signal-to-noise ratio associated with weak target reflections such as those from small anomalies or low-contrast interfaces.
Conventional seismic sources, although dominated by impulsive techniques in practice, also include certain types of mechanical vibrator sources designed to generate frequency-variable sweep signals, typically in the frequency range of 5 Hz to 200 Hz. Vibrator sources of this type are designed and used for deep seismic probing applications in oil and gas exploration and, hence, are made large and powerful to generate the needed source energy. Large vibrator sources of this type employ hydraulic actuator techniques to generate vibrational forces typically in excess of 10,000 lbf which may be coupled to the ground either in vertical or horizontal orientations to generate seismic compressional waves or horizontally polarized shear waves. Such hydraulically actuated sources are dependent on fluid dynamic pressure and flow and, because of their high energy, require relatively large and massive ground-coupling base plates. For these reasons, present- day conventional controlled-waveform vibratory seismic sources have inherent limitations in generating controlled-waveform vibrations at frequencies above about 200 Hz. Sources of this type are also expensive and technically impractical for use in applications where small size is critical for access to the field survey site and/or where higher frequency operation is required for shallow-depth high-resolution seismic surveys.
Alternative force generating techniques for use as actuators in controlled-waveform seismic sources include electromechanical force transducers employing either piezoelectric or magnetostrictive force generation or electrodynamic force transducers utilizing a movable current-carrying coil in a magnetic field. These electrically powered transducer techniques are typically more compact than hydraulic methods and are potentially more accurately controllable in their operation as high-frequency seismic sources. Controlled-waveform seismic signals having high-frequency spectral content up to 1,600 Hz and higher are potentially obtainable using such alternative force generating techniques.
There is a growing need and an emerging commercial market for effective capabilities related to shallow high-resolution seismic surveys directed primarily toward shallow resource exploration, subsurface environmental surveys, and indirect sensing and detection of ground geotechnical conditions and anomalies. In comparison with existing technology, new seismic source techniques and systems appropriate for these shallow applications would preferably operate at relatively high frequencies (typically 200-1,600 Hz) in order to resolve relatively small target details and need only be capable of operating at relatively low dynamic forces (typically 100-1,000 lbf) to provide useflul results at shallow depths and associated short propagation path lengths. Seismic vibrator sources having these operating characteristics may be deployed and operated to generate either vertically oriented or horizontally oriented forces on the ground surface.
When the vibrator is set up to generate predominantly vertical forces on the ground, the radiated seismic waves will consist of compressional (P) waves, vertically polarized shear (SV) waves, and a combination of these P and SV waves known as Rayleigh waves (surface waves). When the vibrator is set up to generate predominantly horizontal forces on the ground, the radiated seismic waves consist of horizontally polarized shear (SH) waves which radiate in the direction normal to the direction of the horizontal force and compressional waves and Rayleigh waves radiated in the direction parallel to the horizontal force. Vibrator sources of the type that generate horizontally oriented ground forces and radiate horizontally polarized shear waves have been discussed in detail in U.S. Pat. No. 6,119,804, which is incorporated herein for all purposes. Therefore, the object of this invention is to create a seismic vibrator source capable of generating vertically oriented forces in the ground to produce controlled seismic waveforms at frequencies typically up to 1,600 Hz and operating at moderate driving forces typically up to 1,000 lbf. This new seismic vibrator source is appropriately matched in size, cost, and mobility to applications in shallow geophysical and geotechnical field surveys.
The controlled-waveform vibrator invention described herein consists of one, two, or more electromechanical force drivers operating either on the force generating principle employing piezoelectric or magnetostriction technology or the force generating principle employing electrodynamic force transducers acting on a frame comprised of a baseplate rigidly coupled to the ground medium and reacting against an inertial mass consisting of a compliantly supported part of the frame so as to impart vibratory dynamic forces to the baseplate and to the ground. In application, the baseplate is placed in firm contact with the ground surface such that the force generator is oriented in the vertical direction. The vertical force applied to the ground produces vertical particle motions in the ground medium and thereby generates seismic compressional waves (P waves) having particle motions normal to the plane of the baseplate which radiate into the body of the medium. Through inherent coupling between the vertical motions of the finite-size baseplate and the vertical shear forces produced at the edges of the plate, vertically oriented shear waves (SV waves) are also generated and radiated into the body of the medium simultaneously with the P waves. This source configuration, through its coherent generation of P waves and SV waves, also generates well-known Rayleigh waves ( also referred to as xe2x80x98surface wavesxe2x80x99) which are essentially confined to propagate only along the surface of the ground medium.
With the use of appropriate masses and compliant suspension springs in this composite vibration transducer assembly, the vibrator operating frequency range can be adjusted to have an upper limit of 1,600 Hz or higher. Further, by arranging the force driver units and reaction masses in an axisymmetrical alignment within a close-fitted frame and arranged with approximately uniform mass distribution within the frame, the tendency of the vibrator to produce nonuniform force distributions at the interface between the ground-coupling baseplate and the ground medium will be minimized. Thus, any undesirable components of dynamic force and motions, such as rocking or tilting, of the vibrator frame or baseplate which could cause horizontally polarized shear waves to be radiated in the medium will also be minimized.
The dynamic force driver unit or units are excited by one or more power amplifiers operating in the audio frequency range at a power level appropriate to drive the overall vibrator system to a total operating force of up to 1,000 lbf when coupled to a ground medium. Excitation signals applied to the vibrator driver may either be a swept-frequency sinusoidal time function having prescribed predetermined parameters of time-dependent frequency sweep, upper and lower frequency limits of sweep range, sweep time duration, and amplitude-time dependence, or alternatively a pulsed swept-frequency sinusoidal time function (commonly termed a xe2x80x98chirpxe2x80x99 signal) having predetermined amplitude, time, and frequency parameters, or alternatively a gated sinewave pulse having a predetermined frequency, pulse amplitude envelope, and repetition time period which may either be periodic or randomly timed, or alternatively a random noise function having predetermined statistical parameters. The predetermined parameters of these excitation signals govern the amplitude, frequency, and time characteristics of the radiated seismic waves.
The essential features of the seismic wave vibrator source disclosed herein are: (1) a baseplate and frame having a means for firmly and rigidly coupling the vibrator to the medium in which vertical dynamic forces produced by an integral electromechanical force driver generate seismic P waves in the ground; (2) a means by which one, two, or more dynamic force driver units are attached to the frame in a manner such that their forces are efficiently transmitted as purely vertical forces to the ground coupling interface via the baseplate; (3) one, two, or more dynamic force driver units, operating either on the piezoelectric or magnetostriction force generating principle or on the electrodynamic force generating principle, to mechanically excite directed forces on the baseplate in the desired seismic source vibrator frequency range; (4) one, two, or more inertial reaction masses suspended on the coupling frame by compliant springs and/or other isolation materials or components, these masses serving as inertial masses against which the force driver units react to apply dynamic forces to the frame and ground coupling baseplate; and (5) frame components constructed integrally with the baseplate to provide accurate and robust support of the reaction masses and compliant springs so as to avoid any tendencies for unwanted static deflections of the reaction masses or unwanted dynamic vibrations or tilting motions of the frame or baseplate during vibrational operation.
As a particular consequence of the finite physical size of the seismic wave vibrator described above, the essential components of the vibrator may be considered to be lumped-constant constant elements typically having the features of rigid mass, compliant spring, absorbent damper, and ideal force generation by electrical-to-mechanical transduction. To describe the energy absorbing characteristics of the earth or other medium to which the vibrator is coupled, this medium may also be represented as a network of effective lumped-constant masses, effective lumped-constant compliances, and effective lumped-constant absorptive dampers all of which interact to characterize the effective mechanical impedance of the medium including the storage and dissipation of vibrational energy in the medium and including the effects of energy lost from the vibrator system in the form of radiated seismic waves. These components comprise a multi-element network of mechanical spring-mass-damper elements which exhibits a mechanical transfer function by which the dynamic forces generated by the force driver units are converted to forces in the ground underlying the coupling baseplate and frame. The physical configuration and dynamic interactions of these components and the lumped-constant elements of the vibrator source govern the frequency response and transfer of mechanical power from the input force drivers to the radiation load in the ground medium.